Pilots flying an aircraft at night and using a night vision imaging system (NVIS) such as night vision goggles (NVGs) may have a field of view that is essentially mono-chromatic green. NVGs in use today are generally generation III, Type I and Class B/C (described in MIL-STD 3009). When pilots wearing NVGs view an approaching aircraft operating in a visible mode, both starboard and port position lights appear green. When not wearing NVGs, pilots would see a green color of a green starboard position light and a red color of a red port position light.
Because red and green position lights seen as red and green in a visible spectrum have different amounts of infrared (IR) content, the (visible light) red and green position lights, when viewed with NVGs in an infrared spectrum, also represent a difference. However, the difference is associated with apparent luminance or how bright the red and green position lights appear rather than a color difference. This apparent luminance is more correctly known as NVIS radiant intensity (NRI). NRI is defined as a radiometric energy of a light source integrated with a response curve of one or more NVGs.